Such apparatuses are used, among other things, in an implantable defibrillation system 101, as that known from U.S. Pat. No. 5,531,766, for example, and illustrated in FIG. 1 herein. In order to stimulate the heart H of a patient P, a defibrillator 102 is electrically connected to a lead 103, which in this example carries a special electrode in the form of a shock electrode 104 which is located at the distal end thereof and placed in the patient's heart. Defibrillation typically takes place in a monopolar manner using a current path between this electrode 104 and a counter electrode 105, which is located close to or removed from the heart H to be stimulated. For example, the housing of the defibrillator 102, which accommodates the units for detecting heart signals and for generating electric pulses, may act as a counter electrode 105.
In another possible embodiment, FIG. 2 shows the distal end of a lead 201. For implementing what is referred to as a bipolar cardio version, the tip 202 of the lead comprises two electrodes 203 and 204. While the electrode 203 forms a contact at the tip of the lead 201, the electrode 204 is designed as an annular contact on the circumference of the lead. The feed lead comprises two helically wound conductors 205 and 206, each of which electrically connects the electrodes 203 and 204 to a connector plug (not shown) at the proximal end of the lead.
At present, patients wearing a conventional defibrillation system according to FIG. 1 cannot undergo magnetic resonance imaging (MRI) examinations, because the strong alternating electromagnetic fields may produce considerable heating of the tip of the lead and resultant damage to the surrounding tissue.
FIG. 3 illustrates a typical temperature curve of the tip of a lead of a defibrillation system in a magnetic resonance imaging unit. As the electromagnetic field (denoted with 301) is activated, the temperature of the tip of the lead rises rapidly, wherein the degree of the increase and the maximum temperature occurring are highly dependent on the lead position relative to the electromagnetic field of the magnetic resonance imaging unit. When the alternating electromagnetic field (denoted with 302) is deactivated, the tip of the lead cools off relatively quickly due to the comparatively low thermal capacity thereof.
In order to solve the heating problem described above, U.S. Publication No. 2009/0105789 proposes the use of a temperature-dependent switch, which disconnects the tip of the lead from the feed lead upon heating. Such a switch, however, results in disadvantageous design properties of the lead, particularly in rigidity of the electrode. Furthermore, the thermal switch exhibits considerable hysteresis, the temperature range is difficult to set, and the tip of the lead can only be completely connected or disconnected, whereby in the disconnected state no stimulation of the heart or conductance of signals is possible.
The problem of the electrode surfaces overheating also occurs with the temporary use of electrodes, electrically active catheters, and metal, partially insulated guide wires in the MRI unit. The application of the MRI unit, for example as part of electrophysiological examinations (mapping), is therefore limited accordingly.
The present invention is directed toward overcoming one or more of the above-identified problems.